1. Field of the Invention
The present invention relates to an imaging device having photosensors forming imaging pixels arranged into an array in an active pixel region provided on a semiconductor chip.
2. Description of the Related Art
Hitherto, silver cameras and digital still cameras have been equipped with an automatic exposure (AE) system for automatically metering the intensity of light to determine an exposure time.
Photodiodes are widely used as a sensor for use in an AE system for metering the intensity of light (hereinafter referred to as an “AE sensor”).
Meanwhile, recent cameras have been equipped with an autofocus (AF) system having an auto-focusing capability, and require a multi-segment AE sensor in order to handle an increasing number of focal points. A sensor having an AF capability is hereinafter referred to as an “AF sensor”.
In order to accommodate an AF sensor having a certain number of focal points, therefore, a specific multi-segment AE sensor is further required.
Furthermore, photodiodes are black-and-white sensors, which hinders a user from knowing color information.
In order to address such inconveniences, AE sensors capable of determining multi-segment information and color information using color CCD (charge-coupled device) solid imaging devices (CCD color sensors) have been on the market recently.
However, the CCD color sensor of this type becomes more multi-segmental than usual, and therefore provides further color information, leading to a heavy load on a CPU (central processing unit) in order to achieve the optimum automatic exposure using such information. Another dedicated CPU is thus required.
For this reason, such a CCD color AE sensor can only be incorporated in specific high-end cameras or high-end digital still cameras.
The configuration and operation of a CCD color AE sensor in the related art are described below in conjunction with a specific example.
FIG. 4 is a block diagram of a CCD color AE sensor in the related art.
As shown in FIG. 4, photosensors 100 are formed into an array in a two-dimensional manner, i.e., vertically and horizontally, on a semiconductor chip, and a color filter (not shown) is provided on the top layer of each of the photosensors 100 for providing color information.
There are also provided vertical transfer (V) registers 101 which are CCD registers for transferring a charge photoelectrically converted by the photosensors 100, and a horizontal transfer (H) register 102 which is a CCD register for horizontally transferring the signal charge received from the V registers 101 to a floating diffuser (FD) unit 103 for converting the signal charge into a voltage signal.
In the FD unit 103, the signal charge transferred from the H register 102 is converted into a voltage signal in response to the transfer operation. An output circuit 104 outputs the voltage signal converted by the FD unit 103 to an external unit.
A SYNC pulse circuit 105 outputs a SYNC pulse for notifying a subsequent A/D circuit about the position of the output signal.
In general, an output timing of AE sensors is determined according to an output instruction from an external unit, and a pulse is therefore necessary for notifying a subsequent circuit about the position of the output signal. The SYNC pulse of the SYNC pulse circuit 105 is used to notify the subsequent circuit about the output signal position.
As indicated by arrows in FIG. 4, the signals are sent to external units from the output circuit 104 and the SYNC pulse circuit 105.
FIG. 5 is a timing chart showing the operation of the related art AE sensor shown in FIG. 4, depicting pulses necessary for the operation of the AE sensor, an output waveform output from the AE sensor, and a SYNC pulse notifying a subsequent A/D circuit about the position of the output signal.
In FIG. 5, a clock signal 106 is a master clock signal input to the AE sensor. The AE sensor internally generates pulses 107, 108, and 110 to 113 based on the clock signal 106 according to external instructions.
The pulse 107 is a read-out-gate (ROG) pulse which drives a charge to be transferred to the V registers 101 from the photosensors 100.
The pulse 108 is a shutter (SHUT) pulse which drains an unnecessary charge from the photosensors 100. When the pulse 108 is high, the charge is drained.
The pulse 110 is a V register pulse which drives the V registers 101 to transfer a charge generated by the photosensors 100.
The pulse 111 is an H register pulse which drives the H register 102 to transfer to the FD unit 103 the charge received from the V registers 101.
The pulse 112 is an output waveform output from the output circuit 104.
The pulse 113 is a (SYNC) pulse for notifying a subsequent circuit about the output position of an active pixel.
The operation of the AE sensor is now described with reference to FIGS. 4 and 5.
The period during which a charge is stored in the photosensors 100 is defined by a pulse interval 109 between the φROG 107 and φSHUT 108.
The charge stored in the period 109 is read out by the V registers 101 when the φROG 107 is high.
When V registers 101 are switched from the high level to the low level, the charge read out by the V registers 101 is transferred to the H register 102. All charges are transferred from the H register 102 to the FD unit 103.
To the H register 102 which is now empty, a charge is again transferred from the V registers 101, and all charges are transferred from the H register 102 to the FD unit 103 again.
In this way, all sensor outputs are transferred to the FD unit 103.
The FD unit 103 converts the received charge into a voltage signal, and outputs the resulting signal in the output waveform (Vout).
Since an output of the AE sensor is usually desired at a desired timing by a photographer, the SYNC pulse 113 is output in a synchronous manner with the output signal so as to notify a subsequent A/D circuit about the output signal position.
The subsequent A/D circuit retrieves an output at this SYNC pulse timing. It is therefore necessary to output the SYNC pulse 113 so as to always correspond to an active pixel and an optical black (OPB) pixel (black reference pixel).
However, such an AE sensor in the related art requires a high performance CPU for a subsequent circuit, and can only be incorporated in specific high-end cameras or high-end digital still cameras. Therefore, there is a problem that such an AE sensor cannot be incorporated as a versatile component in a wide variety of devices including low-end models.